Recently, in the field of audio recording, DAT (digital audio tape recorder) has appeared, and in the VTR field, a 8-mm width standard narrower than a conventional 1/2 inch standard has appeared and is getting more popular. Under such circumstances in the magnetic recording medium, there is demanded increasingly higher density recording as well as a high performance magnetic recording medium of excellent electromagnetic conversion properties and running durability. In order to meet the foregoing demands, fine ferromagnetic powders have come into use for improving the electromagnetic conversion properties of the magnetic recording medium, but the fine ferromagnetic powders do not have good dispersibility in a magnetic coating solution, which results in causing various problems; that is, (1) the electromagnetic conversion properties are liable to be deteriorated; (2) an insufficiently dispersed portion of the ferromagnetic powder is liable to drop off while tape is running; (3) a portion of particles dropping off while running tends to stick to a magnetic head, which causes a temporary output drop; (4) surface smoothness of the magnetic recording medium is impaired, which is liable to cause troubles such as head clogging and edge folding resulting in a deteriorated running durability.
Therefore, there is expected to become available a magnetic recording medium of excellent running durability and electromagnetic conversion properties, which are attributable to excellent dispersibility of the ferromagnetic powder.
Aliphatic dicarboxylic acid such as adipic acid and sebacic acid are conventionally used for a polyesterpolyol component of a urethane resin used as a binder for a magnetic layer. Diols used for the polyesterpolyol component are ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol 1,5-pentanediol, 1,6-hexanediol, and neopentyl glycol. Aromatic dicarboxylic acid is used to improve rigidity of the urethane resin. It is known to introduce a functional group into the urethane resin with aromatic dicarboxylic acid having a functional group such as metal sulfonate.
However, aromatic dicarboxylic acid is liable to make the urethane resin less soluble, which in turn results in lowering dispersibility of the ferromagnetic powder.
It is possible to increase a density of a functional group in the urethane resin and to decrease a molecular weight thereof in order to improve the solubility and dispersibility thereof.
However, the favorable results will not be able to expect as far as aromatic dicarboxylic acid and conventional glycol are used.